How do I reclaim my flat?

If he has stayed in a London flat rent and bills free, of course he is comfortable. He is an adult now and needs to understand that brings responsibilities, like paying your way. While he has been living in your flat buckshee, has he been saving for his own place? If not, then he is taking advantage and grandson or not, needs telling. It needs sorting and soon.

Even better, sell your flat.

Why should the OP do this? She said the arrangement was for a month or so. Also says DH uses the flat when he works in London.

Sounds rather like DS and GS thought that once he was in there, he would be allowed to stay on indefinitely.

Not really fair.

When it comes to my grandchild, I am a sift touch. I would get him stay and charge him rent.

It is very likely that he will struggle to find anything affordable in London unless he is earning a large salary. The average for a share is £850 pm. If you are only using the flat occasionally, what is the problem with him living there and paying you rent?
Even better, sell your flat. You clearly don’t use it that often and a 2 hour commute every now and again isn’t that difficult, and allow someone who needs to live and work in London the opportunity to do so. Maybe even your grandson!

You need to be upfront with him about finding a flat-share - assuming he can’t afford to rent on his own. So many young people do it, and there are plenty of websites listing available places. So I’d give him a deadline and tell him that in the meantime he needs to be paying a reasonable rent.

But IMO it’s his parents who should be having this conversation with him first. Are you on the sort of terms where you could bring this up?

Apart from anything else, if he’s paying nothing, it’s going to come as that much more of a shock when the cost of a roof over your head really hits him.

Speak to your son asap. Your GS should at the very least be paying you rent. Your expenses continue regardless of who is there. However that is not the only issue, your flat has been ' taken over' and is no longer available for your own use. Please take courage and address this quickly.

What would be the problem with charging him a reasonable rent?

My first thought was that you'll probably have to find him a flat or houseshare! He's too comfortable in yours.

But do talk it out with him. He might be feeling quite guilty about still being there.

And charge him, as others have suggested.

It’s not the same thing at all but when my offspring came home to live a couple of times I gave her an achievable deadline of, for example, two months ahead, to find a place. It worked both times with no issues.
In your case maybe he won’t enjoy sleeping on a blow up bed & when you mention money that will hopefully focus his mind.
He’s working & living in an adult world & presumably likes to be treated as an adult? Now is his time to shape up & ship out & show you all how grown up he can be.
Have a pow-wow, set the deadline & keep reminding him about it. He’ll probably surprise you? I hope so! Good luck.

Tell him you want your flat back, that it was only ever meant to be a temporary arrangement, and that in the meantime (which will be short, he has to pay for the utilities.

In other words, be blunt with him about reality.

You will need to have a conversation with your son/grandson because from
his point of view he is very comfortable where he is.
In the long term you will not do him any favours because if he is virtually rent free he will not learn that accommodation costs quite a lot, especially in London.
He needs to share the expenses at the very least. You could ask if he had used this opportunity to save some money, but am betting he may not have done.
He has been very lucky but fir his sake this can’t go on.